2004R Swap

Building a hot motor and dropping it into your muscle car isn’t the only way to add some punch. Upgrading the stock 3-speed slushbox to a 4-speed overdrive will add more than just higher top-end speed; the closer ratio lower gears, will add quicker acceleration and the lock-up feature of the torque convertor will increase fuel economy.

The GM 2004R and 700R4 overdrive transmissions are well known for their ability to handle reasonable amounts of torque and horsepower. The 700R4 is the most common and easiest to find, but the 2004R is actually stronger. While a 700R4 can be swapped into just about anything running a Chevy motor, the same cannot be said for a Buick, Olds or Pontiac. For these cars, particularly the A-bodied musclecars like the Skylark\GS, Cutlass\442, and LeMans\GTO models, the 2004R will bolt up. The BOP (Buick-Olds-Pontiac) bellhousing bolt pattern differs from the Chevy pattern, noted by the valley between the two upper mounts for BOP, where as the Chevy has a peak. 2004R cores are available in either BOP or dual-pattern mounts, allowing the tranny to mount to all Chevy motors as well as the BOP brands. The 2004r is commonly found in intermediate and fullsize cars from 1981 through 1990.

For our 1971 Buick GS convertible, we found a dual-pattern 2004R and rebuilt it using a Trans-go reprogrammer kit, Eagle red-oxide performance clutches, and a Sonnax 4th-gear billet servo. The transmission was built with the intent on putting as much as 500 horsepower in front of it. To transfer the power from the flexplate to the tranny is a TCI Breakaway lock-up convertor with an approximate 2500-rpm stall.

The install is pretty simple, in the GM A-body, the factory crossmember simply needs to be moved to the TH-400 location (about 4 inches back from the TH-350 location), the driveshaft should even be the right length. There are, however, a few items that make the operation of the transmission a little easier. The TV cable is perhaps the single most important aspect of an overdrive transmission. On both the 700R4 and 2004R, the TV (Throttle-Valve) cable takes the place of both the kickdown cable and vacuum modulator. The TV cable operates directly off of the carburetor and controls a solenoid inside the transmission. This solenoid controls when, and to what gear, the transmission downshifts under partial and full throttle. If this cable is not set up correctly, the tranny will never shift right, leaving you the not-so-good feeling of making a mistake buying an overdrive. The overdrive should shift from 4th to 3rd, 2nd and 1st, 3rd to 2nd and 1st and 2nd to 1st under specific conditions and should NEVER shift back and forth between 4th and 3rd.

The problem with these cables comes from the swap itself. When you swap an overdrive to another vehicle, chances are that you are not using the same motor with the correct carburetor or throttle body. When using any set up other than the stock original unit for the transmission, the TV cable geometry is altered. To correct this, we used a kit from Bowtie Overdrives. This very unique unit comes complete with the carburetor adapter for your carb, a mounting plate correctly placing the cable (which we needed desperately on our Buick 350), and the cable itself. The set up process is simple, but does require a pressure gauge be installed in the transmission to complete the set up.

The other issue with an overdrive transmission is the lock-up feature. Swapping the 2004R into the GS required some wiring. There are two ways to accomplish this task; wire the A-pin (top left pin on the 4-pin plug) to ignition hot, leaving the convertor locked up all the time or installing the $109 TCI lock-up kit like we did. The lock-up kit installs in about 30 minutes, and keeps the convertor from locking up until 4th gear, but allows you to add a switch to lock it up manually if needed, such as towing applications (NEVER tow with the tranny in overdrive, it is a sure-fire way to destroy the tranny in short order).

With the tranny buttoned up and all the peripherals installed, the tranny slid right into place. Check out the pics for the details.

1. The Bowtie Overdrive kit is simple, and takes the guesswork out of setting up the TV cable system in a GM overdrive tranny. Don’t forget to order the throttle cable bracket. We did and had to wait.

2. The BTO kit requires a new, longer spring be installed in place of the shorter stock spring. If you skip this step, the tranny won’t shift right. This assembly actuates the piston in the center, which operates the TV system. This bracket must be removed to get to the spring.

4. Push the piston in and pop it, this will force the piston to come out. It took us about five minutes to get the piston out.

5. The spring on the right is the one we want, it is considerably longer than the stock spring.

6. While we were in the pan. We cut the two wires on the lock up solenoid shown here. The TCI Lock-up kit comes with these handy threaded splices.

7. We also replaced the 4th-gear switch with a 2-prong unit that came with the TCI kit.

8. The original lock up wire harness clip use to be mounted to this boss. Use a 3\16” drill bit to make a new hole for the TCI vacuum switch.

9. The vacuum switch needs a manifold vacuum source to operate the lock up servo.

10. The shifter conversion kit we got from Shiftwerks comes with this new shifter mount. It mounts just like the original.

11. When we reinstalled the pan, we installed the Shiftwerks cable mount to the bottom of the pan.

12. The backside of the cable mount should measure 5 3\4” to the center of the cable pin, mounted in the shifter bracket, and with the tranny in overdive. Got all that?

13. Once the tranny was buttoned up, the TV cable was installed. The wire hook from the inside of the tranny slides inside the hole on the end of the TV cable, don’t let it fall off while dropping the housing on the case.

14. The next step is to fill the convertor with a quart of fluid and slide it onto the pump input. The distance from the flat side of the bellhousing to the torque convertor bolt pads, should be in excess of one-inch, otherwise, you won’t have enough clearance for the flexplate. Once clear, bolt it up to the engine and install the 3 flexplate-to-convertor bolts.

15. The BTO cable mount is specific for the type of carb you are running, we are using a Barry Grant Demon. The mount is disassembled and placed over the carb’s throttle mount.

16. Then the front cover is installed with the 3 bolts.

17. The BTO TV cable bracket mounts underneath the carb and the cable snaps inside the hole. The D-shaped button presses in and allows you to slide the cable fore and aft, to accommodate your engine\carb setup.

18. The cable slides into the fitting on the bracket as shown. This bracket corrects the geometry for the cable, giving you the correct amount of tension on the cable when you are supposed to have it. There is a lot of adjustment available here.

19. This is the pressure port, attach a pressure gauge before you test drive the car and check your pressure. The tranny pressure should increase instantly with just a slight pull of the TV cable, that’s how you know it is working.

20. The tranny crossmember in our 1971 Buick GS convertible just needed to be slid back to the TH400 position, noted by the 2 holes to the right on this picture. This represents about 4-6 inches of rearward movement.

21. To finish up the install, we installed the new shift detent that came in the Shiftworks kit. The old detent was unbolted (2 bolts) and the new one bolted up in its place.

22. Comparing the old detent to the new, the new unit has 2 more stops, for the addition of the 4th gear and all the detent are in the correct place when used with the new supplied cable. Don’t try to use your old cable, it won’t put things in the right place, which can destroy a transmission.

A life-long gearhead, Street Tech Magazine founder and editor Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 5 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced. You can follow Jefferson on Facebook (Jefferson Bryant), Twitter (71Buickfreak), and YouTube (RedDirtRodz).